In the developing nervous system, excess neurons are generated which are nonessential, or inappropriately connected, and are eliminated by programmed cell death. In peripheral neuronal populations, the extent of apoptosis is governed by both a limited supply of survival-promoting neurotrophic factors supplied by targets of innervation and by apoptosis-inducing competition factors secreted by neurons that successfully compete for neurotrophic factors, or winning neurons. One of the prevalent families of neurotrophic factors is the glial cell line-derived neurotrophic factor (GDNF) family ligands (GFLs), which support the survival and axon guidance of autonomic, somatosensory and spinal motor neurons. Recently we have discovered that GFL activation of their common signal transducing receptor tyrosine kinase, Ret, causes the association of Ret with p75, a member of the TNF family of death receptors, in sympathetic and sensory neurons. Importantly, we discovered that p75 was critical for GFL-mediated activation of Ret and survival of sensory neurons in vitro. Remarkably, we also found that Ret interacts with p75 upon BDNF stimulation of sympathetic neurons, which triggers the apoptotic death of these neurons. Ret deletion impaired BDNF-induced apoptosis of sympathetic neurons. Our overarching hypothesis is that the p75-Ret receptor complex is a switch regulating survival and apoptosis, depending upon which ligand promotes the assembly of this complex, and serves to integrate coincident survival and death signals. The objectives of this application are two-fold: (1) to test the hypothesis that p75 is critical for the GFL-mediated survival of nociceptive neurons; (2) to test the hypothesis that Ret is necessary for the apoptotic function of the death receptor p75 in sympathetic neurons. In order to accomplish these objectives we will use a combination of biochemical and cell biological techniques in primary neurons and transgenic mice. These experiments are critical for delineating the molecular mechanisms by which the opposing actions of neurotrophic factors and competition factors sculpt peripheral sensory and autonomic circuits. Furthermore, the determination of these receptor mechanisms that create an equilibrium between survival and death will enable a more rational approach for the development of therapeutic strategies for nervous system injuries and diseases.